I was having a discussion with a colleague the other day about this hypothetical situation. Consider this pseudocode:
public void Main()
{
MyDto dto = Repository.GetDto();
foreach(var row in dto.Rows)
{
ProcessStrings(row);
}
}
public void ProcessStrings(DataRow row)
{
string string1 = GetStringFromDataRow(row, 1);
string string2 = GetStringFromDataRow(row, 2);
// do something with the strings
}
Then this functionally identical alternative:
public void Main()
{
string1 = null;
string2 = null,
MyDto dto = Repository.GetDto();
foreach(var row in dto.Rows)
{
ProcessStrings(row, string1, string2)
}
}
public void ProcessStrings(DataRow row, string string1, string string2)
{
string1 = GetStringFromDataRow(row, 1);
string2 = GetStringFromDataRow(row, 2);
// do something with the strings
}
How will these differ in processing when running the compiled code? Are we right in thinking the second version is marginally more efficient because the string variables will take up less memory and only be disposed once, whereas in the first version, they're disposed of on each pass of the loop?
Would it make any difference if the strings in the second version were passed by ref or as out parameters?
When you're dealing with "marginally more efficient" level of optimizations you risk not seeing the whole picture and end up being "marginally less efficient".
This answer here risks the same thing, but with that caveat, let's look at the hypothesis:
Storing a string into a variable creates a new instance of the string
No, not at all. A string is an object, what you're storing in the variable is a reference to that object. On 32-bit systems this reference is 4 bytes in size, on 64-bit it is 8. Nothing more, nothing less. Moving 4/8 bytes around is overhead that you're not really going to notice a lot.
So neither of the two examples, with the very little information we have about the makings of the methods being called, creates more or less strings than the other so on this count they're equivalent.
So what is different?
Well in one example you're storing the two string references into local variables. This is most likely going to be cpu registers. Could be memory on the stack. Hard to say, depends on the rest of the code. Does it matter? Highly unlikely.
In the other example you're passing in two parameters as null and then reusing those parameters locally. These parameters can be passed as cpu registers or stack memory. Same as the other. Did it matter? Not at all.
So most likely there is going to be absolutely no difference at all.
Note one thing, you're mentioning "disposal". This term is reserved for the usage of objects implementing IDisposable and then the act of disposing of these by calling IDisposable.Dispose on those objects. Strings are not such objects, this is not relevant to this question.
If, instead, by disposal you mean "garbage collection", then since I already established that neither of the two examples creates more or less objects than the others due to the differences you asked about, this is also irrelevant.
This is not important, however. It isn't important what you or I or your colleague thinks is going to have an effect. Knowing is quite different, which leads me to...
The real tip I can give about optimization:
Measure
Measure
Measure
Understand
Verify that you understand it correctly
Change, if possible
You measure, use a profiler to find the real bottlenecks and real time spenders in your code, then understand why those are bottlenecks, then ensure your understanding is correct, then you can see if you can change it.
In your code I will venture a guess that if you were to profile your program you would find that those two examples will have absolutely no effect whatsoever on the running time. If they do have effect it is going to be on order of nanoseconds. Most likely, the very act of looking at the profiler results will give you one or more "huh, that's odd" realizations about your program, and you'll find bottlenecks that are far bigger fish than the variables in play here.
In both of your alternatives, GetStringFromDataRow creates new string every time. Whether you store a reference to this string in a local variable or in argument parameter variable (which is essentially not much different from local variable in your case) does not matter. Imagine you even not assigned result of GetStringFromDataRow to any variable - instance of string is still created and stored somewhere in memory until garbage collected. If you would pass your strings by reference - it won't make much difference. You will be able to reuse memory location to store reference to created string (you can think of it as the memory address of string instance), but not memory location for string contents.
Related
Which one is better in terms of memory? I have always used snippet 2.. Is Snippet 1 better in anyway than snippet 2 (performance,memory) ?
Snippet 1
public void GetListOfString(ref List<string> x)
{
x = new List<string>(){"Dave","John"};
}
Snippet 2
public List<string> GetListOfString()
{
return new List<string>(){"Dave","John"};
}
First of all, your first example should be using out, not ref:
public void GetListOfString(out List<string> x)
The method doesn't care what the incoming value is; it just overwrites whatever was there. Using out ensures that a) the caller is not required to initialize the variable before passing it, and b) the method itself is required to initialize the variable before returning (which will ensure against bugs).
If there is any performance difference at all (and I doubt you could measure one), I would expect the first example to be slower, because it has to pass a reference to a variable. Passing by-reference means there has to be a memory location involved where the method can modify the variable's value. Returning a value is a highly optimized scenario, with the value often even stored in a register. And if the variable isn't passed by-reference, then the compiler may be able to enregister the caller's variable too, for an additional performance gain.
And of course, if data is kept in registers rather than being stored on the stack, that represents a (marginal, inconsequential, completely unimportant) reduction in memory footprint too.
But performance and memory footprint should not be your first concern anyway. The primary concern, and in 99.94% of all code the only concern, is what makes sense semantically and operationally. If the method has a need to modify a caller's variable, then pass by-reference, ref or out as appropriate to the scenario. If not, then pass by-value. Period.
Note that if just one variable of the caller needs to be modified, and the method does not otherwise have to return anything (i.e. would be void), then it is considered a much better practice to let the caller handle modifying the variable, and just return the new value for the variable (i.e. as in your second example).
If and when you come to a point in your code where for some reason, you just cannot achieve some specific and measurable performance or memory footprint goal, and you can prove that using passing by-reference will ensure that you will achieve that goal, then you can use performance as a motivation for passing by-reference. Otherwise, don't give it a second thought.
Snippet 2 is much better in terms of readability and usability.
It is probably also slightly better in terms of performance and memory.
But this is just because the caller is forced to create a new list to even call snippet 1. You could argue that this overhead will be optimized away by the compiler, but don't rely on it.
If you had used out instead of ref for snippet 1, then I would say they are the same in terms of performance and memory.
I can sympathize with someone coming from a different programming language background thinking that snippet 1 would be better, but in C# reference types are returned by reference, not copied like they could be in some other languages.
I am trying to optimize my code and was running VS performance monitor on it.
It shows that simple assignment of float takes up a major chunk of computing power?? I don't understand how is that possible.
Here is the code for TagData:
public class TagData
{
public int tf;
public float tf_idf;
}
So all I am really doing is:
float tag_tfidf = td.tf_idf;
I am confused.
I'll post another theory: it might be the cache miss of the first access to members of td. A memory load takes 100-200 cycles which in this case seems to amount to about 1/3 of the total duration of the method.
Points to test this theory:
Is your data set big? It bet it is.
Are you accessing the TagData's in random memory order? I bet they are not sequential in memory. This causes the memory prefetcher of the CPU to be dysfunctional.
Add a new line int dummy = td.tf; before the expensive line. This new line will now be the most expensive line because it will trigger the cache miss. Find some way to do a dummy load operation that the JIT does not optimize out. Maybe add all td.tf values to a local and pass that value to GC.KeepAlive at the end of the method. That should keep the memory load in the JIT-emitted x86.
I might be wrong but contrary to the other theories so far mine is testable.
Try making TagData a struct. That will make all items of term.tags sequential in memory and give you a nice performance boost.
Are you using LINQ? If so, LINQ uses lazy enumeration so the first time you access the value you pulled out, it's going to be painful.
If you are using LINQ, call ToList() after your query to only pay the price once.
It also looks like your data structure is sub optimal but since I don't have access to your source (and probably couldn't help even if I did :) ), I can't tell you what would be better.
EDIT: As commenters have pointed out, LINQ may not be to blame; however my question is based on the fact that both foreach statements are using IEnumerable. The TagData assignment is a pointer to the item in the collection of the IEnumerable (which may or may not have been enumerated yet). The first access of legitimate data is the line that pulls the property from the object. The first time this happens, it may be executing the entire LINQ statement and since profiling uses the average, it may be off. The same can be said for tagScores (which I'm guessing is database backed) whose first access is really slow and then speeds up. I wasn't pointing out the solution just a possible problem given my understanding of IEnumerable.
See http://odetocode.com/blogs/scott/archive/2008/10/01/lazy-linq-and-enumerable-objects.aspx
As we can see that next line to the suspicious one takes only 0.6 i.e
float tag_tfidf = td.tf_idf;//29.6
string tagName =...;//0.6
I suspect this is caused bu the excessive number of calls, and also note float is a value type, meaning they are copied by value. So everytime you assign it, runtime creates new float (Single) struct and initializes it by copying the value from td.tf_idf which takes huge time.
You can see string tagName =...; doesn't takes much because it is copied by reference.
Edit: As comments pointed out I may be wrong in that respect, this might be a bug in profiler also, Try re profiling and see if that makes any difference.
I am currently working on a very large legacy application which handles a large amount of string data gathered from various sources (IE, names, identifiers, common codes relating to the business etc). This data alone can take up to 200 meg of ram in the application process.
A colleague of mine mentioned one possible strategy to reduce the memory footprint (as a lot of the individual strings are duplicate across the data sets), would be to "cache" the recurring strings in a dictionary and re-use them when required. So for example…
public class StringCacher()
{
public readonly Dictionary<string, string> _stringCache;
public StringCacher()
{
_stringCache = new Dictionary<string, string>();
}
public string AddOrReuse(string stringToCache)
{
if (_stringCache.ContainsKey(stringToCache)
_stringCache[stringToCache] = stringToCache;
return _stringCache[stringToCache];
}
}
Then to use this caching...
public IEnumerable<string> IncomingData()
{
var stringCache = new StringCacher();
var dataList = new List<string>();
// Add the data, a fair amount of the strings will be the same.
dataList.Add(stringCache.AddOrReuse("AAAA"));
dataList.Add(stringCache.AddOrReuse("BBBB"));
dataList.Add(stringCache.AddOrReuse("AAAA"));
dataList.Add(stringCache.AddOrReuse("CCCC"));
dataList.Add(stringCache.AddOrReuse("AAAA"));
return dataList;
}
As strings are immutable and a lot of internal work is done by the framework to make them work in a similar way to value types i'm half thinking that this will just create a copy of each the string into the dictionary and just double the amount of memory used rather than just pass a reference to the string stored in the dictionary (which is what my colleague is assuming).
So taking into account that this will be run on a massive set of string data...
Is this going to save any memory, assuming that 30% of the string values will be used twice or more?
Is the assumption that this will even work correct?
This is essentially what string interning is, except you don't have to worry how it works. In your example you are still creating a string, then comparing it, then leaving the copy to be disposed of. .NET will do this for you in runtime.
See also String.Intern and Optimizing C# String Performance (C Calvert)
If a new string is created with code like (String goober1 = "foo"; String goober2 = "foo";) shown in lines 18 and 19, then the intern table is checked. If your string is already in there, then both variables will point at the same block of memory maintained by the intern table.
So, you don't have to roll your own - it won't really provide any advantage. EDIT UNLESS: your strings don't usually live for as long as your AppDomain - interned strings live for the lifetime of the AppDomain, which is not necessarily great for GC. If you want short lived strings, then you want a pool. From String.Intern:
If you are trying to reduce the total amount of memory your application allocates, keep in mind that interning a string has two unwanted side effects. First, the memory allocated for interned String objects is not likely be released until the common language runtime (CLR) terminates. The reason is that the CLR's reference to the interned String object can persist after your application, or even your application domain, terminates. ...
EDIT 2 Also see Jon Skeets SO answer here
This is already built-in .NET, it's called String.Intern, no need to reinvent.
You can acheive this using the built in .Net functionality.
When you initialise your string, make a call to string.Intern() with your string.
For example:
dataList.Add(string.Intern("AAAA"));
Every subsequent call with the same string will use the same reference in memory. So if you have 1000 AAAAs, only 1 copy of AAAA is stored in memory.
Currently, I am working on a project where I need to bring GBs of data on to client machine to do some task and the task needs whole data as it do some analysis on the data and helps in decision making process.
so the question is, what are the best practices and suitable approach to manage that much amount of data into memory without hampering the performance of client machine and application.
note: at the time of application loading, we can spend time to bring data from database to client machine, that's totally acceptable in our case. but once the data is loaded into application at start up, performance is very important.
This is a little hard to answer without a problem statement, i.e. what problems you are currently facing, but the following is just some thoughts, based on some recent experiences we had in a similar scenario. It is, however, a lot of work to change to this type of model - so it also depends how much you can invest trying to "fix" it, and I can make no promise that "your problems" are the same as "our problems", if you see what I mean. So don't get cross if the following approach doesn't work for you!
Loading that much data into memory is always going to have some impact, however, I think I see what you are doing...
When loading that much data naively, you are going to have many (millions?) of objects and a similar-or-greater number of references. You're obviously going to want to be using x64, so the references will add up - but in terms of performance the biggesst problem is going to be garbage collection. You have a lot of objects that can never be collected, but the GC is going to know that you're using a ton of memory, and is going to try anyway periodically. This is something I looked at in more detail here, but the following graph shows the impact - in particular, those "spikes" are all GC killing performance:
For this scenario (a huge amount of data loaded, never released), we switched to using structs, i.e. loading the data into:
struct Foo {
private readonly int id;
private readonly double value;
public Foo(int id, double value) {
this.id = id;
this.value = value;
}
public int Id {get{return id;}}
public double Value {get{return value;}}
}
and stored those directly in arrays (not lists):
Foo[] foos = ...
the significance of that is that because some of these structs are quite big, we didn't want them copying themselves lots of times on the stack, but with an array you can do:
private void SomeMethod(ref Foo foo) {
if(foo.Value == ...) {blah blah blah}
}
// call ^^^
int index = 17;
SomeMethod(ref foos[index]);
Note that we've passed the object directly - it was never copied; foo.Value is actually looking directly inside the array. The tricky bit starts when you need relationships between objects. You can't store a reference here, as it is a struct, and you can't store that. What you can do, though, is store the index (into the array). For example:
struct Customer {
... more not shown
public int FooIndex { get { return fooIndex; } }
}
Not quite as convenient as customer.Foo, but the following works nicely:
Foo foo = foos[customer.FooIndex];
// or, when passing to a method, SomeMethod(ref foos[customer.FooIndex]);
Key points:
we're now using half the size for "references" (an int is 4 bytes; a reference on x64 is 8 bytes)
we don't have several-million object headers in memory
we don't have a huge object graph for GC to look at; only a small number of arrays that GC can look at incredibly quickly
but it is a little less convenient to work with, and needs some initial processing when loading
additional notes:
strings are a killer; if you have millions of strings, then that is problematic; at a minimum, if you have strings that are repeated, make sure you do some custom interning (not string.Intern, that would be bad) to ensure you only have one instance of each repeated value, rather than 800,000 strings with the same contents
if you have repeated data of finite length, rather than sub-lists/arrays, you might consider a fixed array; this requires unsafe code, but avoids another myriad of objects and references
As an additional footnote, with that volume of data, you should think very seriously about your serialization protocols, i.e. how you're sending the data down the wire. I would strongly suggest staying far away from things like XmlSerializer, DataContractSerializer or BinaryFormatter. If you want pointers on this subject, let me know.
I have heard conflicting stories on this topic and am looking for a little bit of clarity.
How would one dispose of a string object immediately, or at the very least clear traces of it?
That depends. Literal strings are interned per default, so even if you application no longer references it it will not be collected, as it is referenced by the internal interning structure. Other strings are just like any other managed object. As soon as they are no longer reference by your application they are eligible for garbage collection.
More about interning here in this question: Where do Java and .NET string literals reside?
If you need to protect a string and be able to dispose it when you want, use System.Security.SecureString class.
Protect sensitive data with .NET 2.0's SecureString class
I wrote a little extension method for the string class for situations like this, it's probably the only sure way of ensuring the string itself is unreadable until collected. Obviously only works on dynamically generated strings, not literals.
public unsafe static void Clear(this string s)
{
fixed(char* ptr = s)
{
for(int i = 0; i < s.Length; i++)
{
ptr[i] = '\0';
}
}
}
This is all down to the garbage collector to handle that for you. You can force it to run a clean-up by calling GC.Collect(). From the docs:
Use this method to try to reclaim all
memory that is inaccessible.
All objects, regardless of how long
they have been in memory, are
considered for collection; however,
objects that are referenced in managed
code are not collected. Use this
method to force the system to try to
reclaim the maximum amount of
available memory.
That's the closest you'll get me thinks!!
I will answer this question from a security perspective.
If you want to destroy a string for security reasons, then it is probably because you don't want anyone snooping on your secret information, and you expect they might scan the memory, or find it in a page file or something if the computer is stolen or otherwise compromised.
The problem is that once a System.String is created in a managed application, there is not really a lot you can do about it. There may be some sneaky way of doing some unsafe reflection and overwriting the bytes, but I can't imagine that such things would be reliable.
The trick is to never put the info in a string at all.
I had this issue one time with a system that I developed for some company laptops. The hard drives were not encrypted, and I knew that if someone took a laptop, then they could easily scan it for sensitive info. I wanted to protect a password from such attacks.
The way I delt with it is this: I put the password in a byte array by capturing key press events on the textbox control. The textbox never contained anything but asterisks and single characters. The password never existed as a string at any time. I then hashed the byte array and zeroed the original. The hash was then XORed with a random hard-coded key, and this was used to encrypt all the sensitive data.
After everything was encrypted, then the key was zeroed out.
Naturally, some of the data might exist in the page file as plaintext, and it's also possible that the final key could be inspected as well. But nobody was going to steal the password dang it!
There's no deterministic way to clear all traces of a string (System.String) from memory. Your only options are to use a character array or a SecureString object.
One of the best ways to limit the lifetime of string objects in memory is to declare them as local variables in the innermost scope possible and not as private member variables on a class.
It's a common mistake for junior developers to declare their strings 'private string ...' on the class itself.
I've also seen well-meaning experienced developers trying to cache some complex string concatenation (a+b+c+d...) in a private member variable so they don't have to keep calculating it. Big mistake - it takes hardly any time to recalculate it, the temporary strings are garbage collected almost immediately when the first generation of GC happens, and the memory swallowed by caching all those strings just took available memory away from more important items like cached database records or cached page output.
Set the string variable to null once you don't need it.
string s = "dispose me!";
...
...
s = null;
and then call GC.Collect() to revoke garbage collector, but GC CANNOT guarantee the string will be collected immediately.